High fiber count cables are employed for installations that require a large number of fiber optic connections such as data centers or other computer/data transfer intensive applications. In order to address this need, high fiber count optical fiber cables are produced having increased fiber counts, typically in the range of 216 through 432 fibers.
A typical high fiber count cable design includes an outer cable jacket, within which are multiple loose tube fiber cables arranged in layers around a central strength member. For example, a typical outer cable jacket may include eighteen loose tubes (one six tube layer and one twelve tube layer) to thirty six loose tubes (with an additional eighteen tube layer) therein. Within each loose tube fiber cable, another twelve UV coated optical fibers are loosely arranged. Loose tube fiber cables refer to an arrangement where the multiple individual optical fibers inside the loose tubes are substantially un-connected to the tube. Such an arrangement effectively bundles a large number of optical fibers into a relatively condensed outer jacket.
However, the current arrangement for such cables maintains significant drawbacks. Typically, there are two competing interests in the designs of such high fiber count optical fiber cables. The first concern is that users of such cables would like to have mid-span access to a fiber contained within one of the loose tubes within the larger high fiber cable. This is a desirable quality to allow for more complex installations where a particular fiber may need to be accessed mid-span of the larger cable, rather than at the end of the cable where the fibers are more easily accessed. But, because of the general high fiber count construction, mid-span access proves to be difficult.
For example, the lower (inner) layers of the loose tube cables are typically inaccessible due to the stranding of the internal elements. To access a lower layer, obviously a user would need to move the outer layers first. However, as with many cables having internal components, the internal components need to be stranded within the outer jacket for a number of reasons, such as allowing for easy spooling and un-spooling during installation and preventing vast length differentials between internal components when the larger cable is spooled or bent extensively during installation.
A typical stranding for such high fiber count optical fiber cables having a single layer is for a helical stranding arrangement such that all the internal components of that layer rotate helically in a single direction along the length of the cable. In larger high fiber count cables each of the layers are wound in opposite directions. However, as noted above, such a helical stranding makes it difficult and at times impossible to achieve mid-span access of many of the fibers within the lose tubes inside the cable. For example, with high fiber count optical fiber cables having multiple layers of loose tube optical fiber cables therein, mid-span access to fiber requires an un-twisting of the stranded cable, which is unworkable when there are multiple layers of loose tubes, each of different lay length and direction. Additionally, the problem of un-twisting is exacerbated because the loose tubes employ plastics having a high Young's modulus which are particularly stiff.
Prior art attempts to address this problem use an alternative S-Z stranding of the loose tubes within the cable, where the loose tubes reverse stranding direction periodically along the length of the cable. This allows easier mid-span access to the fibers because it is easier to unwind the tubes in the upper layers allowing the user to reach the fibers contained in the inner layers (center of the outer jacket). However, S-Z designs are less robust stranding designs that tend to unravel or un-rotate over time as a result of temperature expansion, and by excessive handling of the cable during installation/relocation. Use of S-Z may incorporate binders to hold the S-Z rotation, but such binders add yet another barrier to mid-span access of a fiber, add cost weight and additional stranding steps, and also results in undesirable crushing stresses on the fibers inside the tubes.
Another related drawback associated with the prior art in high fiber count optical fiber cables is that prior art cables typically use relatively high Young's modulus plastics for the loose tube construction. High Young's modulus plastics provide a greater level of protection to the fibers contained therein. However, in addition to being difficult to unwind for mid-span access due to their inherent stiffness, the use of such high modulus plastics exhibit higher temperature contraction reactions requiring, the tighter lay lengths in response. However, such designs, in addition to making it more difficult to achieve mid-span access in helical stranded cables because of the tighter winding that needs to be unwound, such high modulus plastics impart some inherent fiber bend stresses which is locked in during manufacture by the twisting of the tubes, and may be augmented by the use of water blocking gels that assist in locking in this strain. Such strain increases attenuation in the fibers and further hampers efforts for mid-span access.